In the process to produce ethylene the stream to be purified comprising essentially ethylene, ethane, CO, CO2, H2, CH4, C3+ hydrocarbons as well as oxygenates is sent to a stripper (also referred to as a demethanizer) to produce an overhead stream comprising essentially CO, H2 and CH4 and a bottom stream comprising essentially ethylene, oxygenates, ethane, CO2 and C3+ hydrocarbons. Separation of light components depends on their proportion in the stream. If the proportion of light components is relatively high, a very low temperature will be required to separate them from the ethylene stream, and therefore a cold stream will be required.
The following prior arts have already described purification of olefins but they need a caustic wash or a wash column to remove the oxygenates or both.
US 20030098281 A1 describes a method of controlling water and/or oxygenate concentrations of an olefin stream. The method includes contacting the olefin stream with a liquid absorbent. The liquid absorbent is selected from the group consisting of a polyol, amine, amide, nitrile, heterocyclic nitrogen containing compound, and mixtures thereof. A gaseous stream comprising essentially steam, ethylene, propylene and less than 2 w % of oxygenates is condensed in a quench tower. The overhead of said quench tower is washed with a caustic solution to remove CO2 and then contacted with the liquid absorbent to remove the oxygenates.
WO 03 020670 A1 provides a method for removing oxygenated components such as acetaldehyde, CO2 and/or water from an olefin stream. It explains it is desirable to remove such oxygenated components, since they may poison catalysts that are used to further process olefin composition. In addition, the presence of certain oxygenated compounds, such as acetaldehyde, can cause fouling in other olefin purification units, e.g., acid gas treating units. The method comprises providing an olefin stream containing ethylene, propylene, C4+ olefins and acetaldehyde. The olefin stream is separated into a first fraction and a second fraction, wherein the first fraction comprises at least a majority of the ethylene and/or propylene present in the olefin stream, and the second fraction comprises at least a majority of the C4+ olefins and acetaldehyde present in the olefin stream. The first fraction is then acid gas treated by sodium hydroxide or potassium hydroxide. The olefin stream is separated by distillation, preferably, the distillation is extractive distillation using an extractant. The preferred extractant is a polar composition having an average boiling point of at least 38° C. at 1 atm (0.1 MPa). Methanol is one type of preferred extractant.
WO 03 020672 A1 describes method of removing dimethyl ether from an ethylene and/or propylene containing stream. The olefin stream is passed to a water absorption column, methanol is used as the water absorbent. Methanol and entrained water, as well as some oxygenated hydrocarbon, is recovered as the bottoms stream of said water absorption column, an overhead olefin is recovered and sent to a distillation column. The distillation column separates ethylene and propylene, as well as lighter boiling point components from the dimethyl ether and heavier boiling point components, including C4+ components and methanol remaining from the methanol wash. Additional methanol is added to the distillation column to reduce clathrate and/or free water formation in the distillation column. The ethylene and propylene containing stream exits the distillation column as overhead and the heavier boiling point components which include the dimethyl ether and C4+ components exit the distillation column as the bottoms. Ethylene and propylene then flow to a caustic wash column.
WO 03 033438 A1 describes a method for processing an olefin stream containing oxygenates and water, comprising: providing an olefin stream containing oxygenates and water; dewatering the olefin stream; compressing the dewatered olefin stream; washing the olefin stream with methanol to remove at least a portion of the oxygenate from the olefin stream; contacting the methanol washed olefin stream with water; and fractionating the water contacted olefin stream. The recovered olefin stream (washed with methanol and then with water) is further sent to an alkali wash and a drying step. The olefin stream containing oxygenates and water is the effluent of an MTO process.
EP 0 669 389 describes a cryogenic ethylene recovery system in which the ethylene-containing feed gas is cooled and condensed, prior to fractionation. In particular this document describes an improved method which utilizes a combination of one or more partial condensers followed by one or more dephlegmators. In this document, the purification is performed with the help of two demethanizer. The overhead stream of the first demethanizer 117 is passed directly to the second demethanizer 119 without any other treatment.
U.S. Pat. No. 6,444,869 describes a process for the production of ethylene from an oxygenate conversion effluent stream. The oxygenate conversion effluent stream comprises hydrogen, methane, ethylene, ethane, propylene, propane and C4+ olefins. This effluent is compressed, treated to remove oxygenates, passed to a carbon dioxide removal zone wherein carbon dioxide is absorbed by contacting a caustic solution or by contacting an amine solution in combination with a caustic solution in a conventional manner to remove the carbon dioxide, dried, then fractionation is made through a deethanizer and a demethanizer.
US 2005-0283038 A1 described a process for producing an olefins stream from a first vapor effluent stream from an oxygenate to olefin conversion reaction, said first vapor effluent stream comprising C2 and C3 olefins, C4 hydrocarbons, and C2 to C6 carbonyl compounds. In the process, the temperature and pressure of the first vapor effluent stream are adjusted to produce a second vapor effluent stream having a pressure ranging from about 100 psig to about 350 psig (790 to 2514 kPa) and a temperature ranging from about 70° F. to about 120° F. (21 to 49° C.), said second vapor effluent stream containing about 50 wt. % or more C4 hydrocarbons based upon the total weight of C4 hydrocarbons in the first vapor effluent stream. The second vapor effluent stream is then washed with a liquid alcohol-containing stream to produce a third vapor effluent stream, whereafter the third vapor effluent stream is washed with liquid water to provide a fourth vapor effluent stream comprising the C2 and C3 olefins and about 1.0 wt. % or less C2 to C6 carbonyl compounds. In one embodiment of such a recovery process, at least part of the fourth vapor effluent stream is contacted with a basic component, such as caustic or an amine, to remove the bulk of the carbon dioxide therefrom (thus removing “acid gas” from the fourth vapor effluent stream), whereafter the CO2-depleted stream is dried.